This was the original intent. The increased coverage of the Nikon F series camera allowed for much more accurate framing for scientific, technical and copy work.
As benjiboy noted it's to allow for that bit of masking from the slide mount AND from a standard negative carrier.

When I had a Nikon F2 and shot mainly slides and mounted them for projection, I found the 100% viewfinder very annoying, I know 100% coverage in theory is ideal , but in practice it was a pain in the ass.

01-23-2013, 10:02 AM

narsuitus

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When I was shooting a lot of medical and scientific subjects, flat artwork, and architectural subjects with small format cameras, I often needed the 100% viewfinder accuracy for proper framing. The Nikon F, F2, F3, and F4 provided me with this very useful feature.

However, now that I shoot more vacation/travel subjects, I really prefer cameras (like rangefinders) that have more than 100% viewfinder coverage.